This artist's concept appeared on the Feb. 23, 2017 cover of the journal Nature announcing that the nearby star TRAPPIST-1, an ultra-cool dwarf, has seven Earth-sized planets orbiting it. Two of the planets were discovered in 2016 by TRAPPIST (the Transiting Planets and Planetesimals Small Telescope) in Chile. NASA’s Spitzer Space Telescope and several ground-based telescopes uncovered five additional ones, increasing the total number to seven. The TRAPPIST-1 system is located about 40 light-years from Earth.

Astronomers have used NASA's Hubble Space Telescope to conduct the first spectroscopic survey of the Earth-sized planets (d, e, f, and g) within TRAPPIST-1's habitable zone. The habitable zone is a region at a distance from the star where liquid water, the key to life as we know it, could exist on the planets’ surfaces. This spectroscopic survey is a follow-up to Hubble observations made in 2016 of the atmospheres of the inner TRAPPIST-1 planets b and c. Hubble reveals that at least three of the exoplanets (d, e, and f) do not seem to contain puffy, hydrogen-rich atmospheres similar to gaseous planets such as Neptune. Additional observations are needed to determine the hydrogen content of the fourth planet’s (g) atmosphere. Hydrogen is a greenhouse gas, which smothers a planet orbiting close to its star, making it hot and inhospitable to life. The results, instead, favor more compact atmospheres like those of Earth, Venus, and Mars.